The present invention relates to electroless plating processes and in particular to the method of treating non-conductors for the reception of electroless coating thereon.
Methods for the catalytic treatment of non-conductor dielectric substrates receptive for electroless plating are well known in the art.
In the past, two basic procedures have been adopted, both resulting in a catalytic surface capable of electroless plating initiation. The first procedure involves a two step process of immersion into an acidic solution comprising stannous ions followed by an immersion into a solution comprising noble metal ions such as palladium. In the alternate procedure, immersion into an acidic solution containing stannous and palladium ions is made with a second step (acceleration) which is optional.
The latter procedure has been described in U.S. Pat. No. 3,011,920 consisting of an admixture of tin(II) and Pd(II) which exists as a colloidal suspension resulting from the formation of metallic palladium nuclei which is stabilized by the excess tin(II) ions. It is noted, however, that in U.S. Pat. Nos. 3,672,938; 3,682,671; and 3,672,923 the same admixture was found and claimed to be a true solution rather than a colloidal suspension. In the latter patents the tin(II) and palladium(II) react to form an ionic complex product. In U.S. Pat. No. 3,841,881 an improvement related to composition of tin(II) and palladium(II) has been achieved through the extraneous addition of a product derived from separately aging stannic chloride. Several publications are available describing the chemistry which takes place within the aging process and the subsequent chemical interactions; these references are thus made part of this application.
1. N. Feldstein et al, J. Electrochem. Soc., 119, 1486 (1972). PA0 2. N. Feldstein et al, Plating, 59, 140 (1972). PA0 3. N. Feldstein et al, J. Electrochem. Soc., 119, 668 (1972). PA0 4. N. Feldstein et al, U.S. Pat. No. 3,666,527. PA0 5. N. Feldstein et al, J. Electrochem. Soc., 120, 475 (1973). PA0 6. N. Feldstein, "Proc. of the AES Fourth Plating in the Electronics Industry Symposium, "Indianapolis, Indiana (1973). PA0 7. N. Feldstein et al, J. Electrochem. Soc., 120, 875 (1973). PA0 8. N. Feldstein et al, J. Electrochem. Soc., 121, 738 (1974). PA0 9. L. Pytlewski, U.S. Pat. No. 3,890,429.
While there were many advantages associated with the incorporation of the aged tin(IV) product, some disadvantages have been encountered in practical operation. Specifically, it has been observed that the properties of the aged tin(IV) suspension are time dependent (even if the product is kept at room temperature). This behavior is noted in the performance of the product, thereby requiring continuous changes in the actual amount to be used as taught in U.S. Pat. No. 3,841,881. In a recent publication, B. K. W. Baylis et al, J. Electrochem. Soc., 123, p. 349 (1976) (see Table I) the time variant properties of aged tin(IV) are described as related to plating performance. Such variations are highly undesirable, for practical operation, especially whenever inventory of product is kept. In addition, it has been noted that concentrated solutions of tin(IV) are not efficient with respect to the aging process (see reference 1, FIGS. 1 & 2).
The present invention relates to improvement of the catalytic composition for electroless plating. In particular, the present invention reduces the number of concentrate stock solutions required. At the same time, the present invention provides the beneficial effects associated with the aged tin(IV) product and yet significantly minimizes the inherent variations of such product when produced separately (as taught in the prior art).